Geant4 Simulations for the Radon Electric Dipole Moment Search at

distribution, this average intensity can increase or decrease over the depolarization
time. The 416 keV M1 transition shown in Figure 4.5(b) clearly illustrates this effect.
Even though the number of nuclei inside the cell, and hence the total activity is
decreasing, the observed average count rate increases over the depolarization time
scale (which is short compared to the 223 Rn half-life of 24.3 minutes). This change
in the average count rate occurs because the “donut shaped” distribution depolarizes
into an isotropic distribution that has a larger average γ-ray intensity in the plane of
the eight GRIFFIN detectors. This effect is further studied in Figures 4.6 and 4.7.
The remaining terms describe the precession, where A 5 is the frequency, A 6 is the
phase and A 7 , A 8 , A 9 , ··· are the intensities of odd-sine oscillations. A maximum of
14 odd-sine terms were included into the fitting program (up to A 20 ), however, often
only the first few odd-sine terms were non-zero. In this case all other sine terms fixed
to zero and removed from the fit.
Figure 4.8 illustrates the fit to the 416 keV time projection data given in Figure
4.5(b). In the fit the half-life was “fixed” to its simulated value of 1458 seconds
and the remaining parameters were left “free” to be fitted by the program. The fit
resulted in a good reduced χ 2 of 1.04 and a precession frequency which agrees with
the simulated value of 2 Hz. The input precession frequency was 1 Hz, but due to the
symmetric nature of the γ-ray angular distribution the observed frequency is twice
the input value. The fitted parameters are summarized in Table 4.1. It should be
noted that the analyzing power A in Equation 4.1, which that detects a change of
counts ∆N = AN, was estimated as A = 0.2 in the calculated statistical limits for
the RnEDM experiment [32]. Figure 4.8 validates that estimate as the ratio of the
signal amplitude to the background is approximately 0.2.
From this single measurement we can calculate the resulting sensitivity in the
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